Bipolar electrolytic diaphragm cells, useful in the electrolysis of brines, e.g., aqueous solutions of alkali metal halides such as sodium chloride, have a plurality of individual electrolytic cells in bipolar mechanical and electrical configuration. The structure for effecting bipolar mechanical and electrical configuration is an electroconductive, electrolyte-resistant backplate separating the adjacent cells from one another, and serving as a structural member for the cathodes of one cell and the anodes of the next adjacent cell in the bipolar electrolyzer.
The backplate has three functions. First, the backplate separates the catholyte of one cell from the anolyte of the next adjacent cell of the electrolyzer. Second, the backplate is a conductive member connecting the cathodes of one electrolytic cell and the anodes of the next adjacent cell in the electrolyzer, thereby providing bipolar electrical configuration between the cathodes of one cell and the anodes of the next adjacent cell in the electrolyzer. Third, the backplate acts as a common structural member, having cathodes extending substantially perpendicularly from one side and anodes extending substantially perpendicularly from the other side, thereby providing bipolar mechanical configuration.
In the design and construction of bipolar diaphragm electrolyzers, it is particularly important to conduct current from the cathodes of the one cell to the anodes of the next adjacent cell with the minimum voltage drop between cells. This voltage drop is a combination of IR voltage drop and contact resistance voltage drop. This minimization of voltage drop must be accomplished with the minimum of seepage of electrolyte through the backplate from the electrolyte of one cell to the electrolyte of the adjacent cells. The minimization of IR drop through the backplate and the minimization of contact resistance between the cathodes of one cell and the anodes of the next adjacent cell while maintaining the structural integrity of the backplate, are particularly important goals. This is because a typical electrolyzer may contain a plurality of cells, for example, from 3 to 8 or 11 or more cells, for example, as many as 70 or 80 cells. Additionally, electrolyzers are frequently connected in series, thereby providing as many as three or four hundred individual cells in a series. Bipolar electrolyzers frequently operate at high currents; for example, 70,000, 100,000, or even 150,000 amperes. Thus, it can be seen that a voltage reduction of only ten one-thousandths of a volt per cell may result in an overall voltage savings of 3 or more volts across an entire cell circuit and a power savings of as much as three hundred kilowatts across the entire cell circuit.
Early attempts to conduct current from the cathode of one cell to the anode of the next adjacent cell in an electrolyzer with minimum IR and contact resistance voltage drops and substantially no seepage of electrolyte between electrolytic cells generally required means for conducting electricity from a cathode of one cell through the backplate to an anode of the next adjacent cell, and for connecting the anode and cathode to the backplate, which breached the backplate. Such conductor/connectors had a conductive material, for example, copper, sheathed in a catholyte-resistant metal, such as steel, on one side of the copper, and an anolyte-resistant metal, such as titanium, on the other side of the conductor. The titanium sheathing was typically silver welded to the copper conductor using a 99.99 percent pure silver filler, and the steel sheathing was typically welded to the copper conductor using a copper-silicon filler metal. The silver welded joints were characterized by high cost, and a substantial degree of non-reproductibility, thereby necessitating 100 percent inspection of all of the joints. Furthermore, the means provided for complete inspection of all soldered joints were themselves subject to occasional failure, allowing electrolyte to attack the copper conductor, raising the voltage drop across the cell, and ultimately leaking into the electrolyte of the adjacent cell, and causing failure of the conductor/connector.